Project Summary/Abstract Opioid use disorders (OUD) are the second most common type of drug use disorder in the US, with nearly 2 million Americans with prescription opioid- (PO) and ~570,000 with heroin-related OUD. The escalation in OUD during the past two decades has been most pronounced among youth, many of whom demonstrate a rapid transition from nonmedical PO use (16-17 y/o), to heroin (19-20 y/o), with most progressing to injection drug use (IDU), within a year of starting heroin use (20-21 y/o). Progression to IDU is characterized by uniquely high levels of risk for youth, including higher rates of overdose (OD) and HIV and HCV incidence, compared to older peers. Addiction severity, psychosocial functioning, and social networks are robust predictors of transitioning to IDU; however there is virtually no research on how to prevent or halt this transition to IDU. Given the paucity of interventions targeting this large and vulnerable group of youth, we propose to adapt and evaluate an innovative, engaging mHealth intervention to prevent young opioid users (18-29) from transitioning to IDU. Aim 1: During months 1-12, we will adapt our existing mobile intervention for OUD that includes daily text messages plus key components of evidence-based CBT interventions, including Functional Analysis of Drug Use, Self- Management, and Social / Recreational Counseling. New components specific to youth will focus on the role of peers on opioid use and IDU, and OD prevention / response training. Our iterative development process will include focus groups with opioid-using youth (n=24), interviews with important stakeholders (e.g., youth treatment providers; n=6), and feedback and usability data from opioid-using youth (n=30). Aim 2: During months 13-31, we will conduct a small randomized, controlled trial of the tailored mHealth intervention with young opioid users who have not transitioned to regular injection (n=64) and compare (1) assessment plus in-person OD prevention / response training (including naloxone) versus (2) assessment plus in-person OD prevention / response training (including naloxone) plus our mHealth intervention. Feasibility and acceptability will be assessed via participant feedback, retention, and usage data. Diffusion will be defined as the number of participants' peers who download the intervention app for their own use. Preliminary effectiveness will be measured via reductions in opioid use (TLFB, urine / hair toxicology) and self-reported injection status at 4, 8, and 12 weeks, and 3 and 6 month follow-up. Secondary outcomes include HIV/HCV risk behavior, OD, opioid-related problems (e.g., withdrawal episodes), and social network IDU-related norms and behaviors. If results are promising, this novel intervention will be expanded for examination in a large- scale efficacy / effectiveness trial. This intervention may have tremendous impact on improving access, acceptability, and potency of opioid use interventions for youth. Given the extremely widespread use of mobile apps among young adults (98% of 18-29 y/o own mobile phones and spend ~3 hr/day using mobile apps), evidence-based mHealth interventions may have significant reach for this difficult-to-engage population.